In a balanced three-phase power system, the individual phase quantities (voltages/currents) are equal in magnitude and are displaced with respect to each other by 120°. Generally, transmission systems are balanced, but distribution power systems, are highly unbalanced. Some of the sources of imbalances include asymmetrical winding impedances of distribution transformers and asymmetrical line impedances. The main source of voltage imbalance at distribution points is the uneven distribution of single phase loads. Another source of imbalance is a fault or other interruption (e.g. for maintenance) along a single phase line. This would create a need for short term balancing until the affected line can be brought back into service. Further, the energy consumption by these loads changes continuously, making the balancing process challenging.
Effects of phase imbalances include increased line losses and heating, equipment overloading, and decreased system stability. Unbalanced phase currents create neutral current flows (if neutral path exists) leading to additional losses (neutral line losses) on the system. Unbalanced voltages are an issue for three phase loads like induction motors or power-electronic converters, as the negative and zero sequence currents create additional losses. Unbalanced currents also lead to torque pulsations, increased vibrations and mechanical stresses, increased losses, and motor overheating and thus degrades the performance and shortens the life of the induction motors. In power electronic converters, the effects of unbalanced voltages include increased input current distortion, generation of twice the fundamental frequency voltage ripple in the DC link and an increased reactive power.
There is currently a very limited ability to alleviate phase imbalances on distribution systems. Manual feeder switching operations to transfer circuits with multiple customers or loads from one phase to another phase at the substation level is one way of balancing an electricity distribution system. Several algorithms have been developed to optimize feeder switch positions. However, the switching is performed in a discrete manner and it cannot dynamically balance the system load. Another way to balance the electricity distribution system is to utilize passive power filters that balance the load impedances. The load currents in this case are balanced by adding reactive elements in parallel to the loads. An alternative way of balancing currents is to use a shunt connected thyristor controlled static VAR compensator, where again the load current is balanced by adding reactive elements in parallel to the load. These are costly solutions, however, since they require additional capital investments to add equipment to the system. Further, they are not dynamic and may also inject harmonics into the system.
For these and other reasons, there is a need for embodiments of the present invention.